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Suitability of Different Probability Distributions for Performing Schedule Risk Simulations in Project Management 2016 Proceedings of PICMET '16: Technology Management for Social Innovation Suitability of Different Probability Distributions for Performing Schedule Risk Simulations in Project Management J. Krige Visser Department of Engineering and Technology Management, University of Pretoria, Pretoria, South Africa Abstract--Project managers are often confronted with the The Project Evaluation and Review Technique (PERT), in question on what is the probability of finishing a project within conjunction with the Critical Path Method (CPM), were budget or finishing a project on time. One method or tool that is developed in the 1950’s to address the uncertainty in project useful in answering these questions at various stages of a project duration for complex projects [11], [13]. The expected value is to develop a Monte Carlo simulation for the cost or duration or mean value for each activity of the project network was of the project and to update and repeat the simulations with actual data as the project progresses. The PERT method became calculated by applying the beta distribution and three popular in the 1950’s to express the uncertainty in the duration estimates for the duration of the activity. The total project of activities. Many other distributions are available for use in duration was determined by adding all the duration values of cost or schedule simulations. the activities on the critical path. The Monte Carlo simulation This paper discusses the results of a project to investigate the (MCS) provides a distribution for the total project duration output of schedule simulations when different distributions, e.g. and is therefore more useful as a method or tool for decision triangular, normal, lognormal or betaPert, are used to express making. the uncertainty in activity durations. Two examples were used to compare the output distributions, i.e. a network with 10 B. Research objectives activities in sequence and a network where some of the activities are performed in parallel. The results indicate that there is no The main objective of this study was to compare the significant difference in the output distributions when different output of a project schedule risk simulation when alternative input distributions with the same mean and variance values are input distributions are used to express the uncertainty in the used. duration of individual activities of the project. This objective is supported by the following sub-objectives. I. INTRODUCTION Perform a literature search on comparison of probability distributions used in general Monte Carlo simulation. A. Background Define two test ‘projects’, one with a number of activities Many factors influence the total project outcome or to be performed in sequence and one with parallel performance. The three dimensional goal for any project is activities, that can be used to compare different well known, i.e. to finish the project on time, within budget distributions. and with satisfactory performance or quality [11]. Large and Run simulations using different input schedule complex projects typically have high uncertainty in the final distributions for a series and parallel network with a project outcome, e.g. space exploration (International space simulation add-in for MS Excel and standalone simulation station, Mars Curiosity lander), passenger airplane software. development (Airbus A380, Boeing 787) and bridge Compare the total project duration outputs graphically and construction (Akashi-Kaikyo, Millau). Such complex projects through statistical analysis. are often late and exceed the original budget. Good project and risk management therefore involves the reduction of The following hypothesis was defined. uncertainty in the outcome of the project. H0: The distribution for the total duration of a project One method of reducing general uncertainty in the project comprising ten individual activities that each have a duration is to model the uncertainty in the duration of triangular distribution is not significantly different to the individual activities by means of probability distributions and distribution for the total project duration obtained when to run a simulation with multiple trials to produce a the ten activities have a normal, lognormal, Gumbel or probability distribution for the total project duration. An Weibull distribution, 95% confidence level. initial simulation is usually performed before project H1: The distribution for the total duration of a project implementation and for projects with a long duration the comprising ten individual activities that each have a simulation can be repeated at various stages or milestones of triangular distribution is significantly different to the the project. Actual duration values for completed activities distribution for the total project duration obtained when are then used in place of the initial estimates and 80 or 90% the ten activities have a normal, lognormal, Gumbel or certainty of completion are compared with the original values Weibull distribution, at a 95% confidence level. derived from the first simulation. This was the approach used for the schedule risk management of the Øresund bridge project that was completed 5 months ahead of schedule in 2001 [1]. 2031 2016 Proceedings of PICMET '16: Technology Management for Social Innovation II. LITERATURE AND THEORY and ρ is the random number obtained with the RAND() function of MS Excel or the RandUniform(0,1) function A. Project scheduling provided by SimVoi. Planning and scheduling are tools that assist in achieving the three crucial goals of project management. The work Normal distribution breakdown structure (WBS) is one of the outputs of the The normal distribution, also know as the Gaussian planning function and enables the development of the project distribution, is a continuous and symmetric distribution cost breakdown and the project schedule. Single point defined by two parameters, i.e. the mean value µ and the estimates are used for the duration of activities and the standard deviation, . The density function of the normal activity network enables a calculation of the critical path and distribution is defined over -∞ to +∞ and care should be taken total project duration. In modern projects, a schedule when using this distribution in simulation where the variable simulation is often used to complement the single point is only defined from 0 - ∞, e.g. duration of an activity which deterministic approach by incorporating uncertainty in the cannot be negative. The normal distribution is symmetric duration of the activities [2]. around the mean value whereas the other distributions investigated are non-symmetric. Random variates for the B. Project risk simulation normal distribution can be calculated with the MCS methods have become more popular amongst NORMINV(RAND(), µ, ) function of MS Excel or the project managers and planners in the last decade due to the RandNormal(µ, ) function provided by SimVoi. availability of fast desktop computers and software that is freely available, especially as add-ins for spreadsheets like Lognormal distribution MS Excel. The method is well documented and discussed in The lognormal distribution is a continuous, non- various textbooks [11], [2], [14], and [6]. Wood [16] says symmetric distribution that is often used to model the MCS can be “applied in many diverse fields that require duration of activities or tasks. It applies mostly to novice outcomes to be quantified statistically under conditions of artisans or workers that have to perform non-standard and uncertainty to aid in decision-making.” complex tasks. These tasks often have an overflow especially Various software programs for performing simulations are when something goes wrong. It has two parameters, i.e. the available to model uncertainty in the cost or duration of mean value µ and the standard deviation . The random activities. Two standalone software programs that use variates can be calculated using the MS Excel function discrete event simulation are Arena and GoldSim. Some add- LOGNORM.INV(RAND(), µ ,) or the RandLognormal(µ, ins for performing simulations with MS Excel are @Risk, ) function provided by SimVoi. Crystal Ball, SimVoi and RiskAmp. GoldSim [5] and SimVoi [15] were selected for this research project. Gumbel distribution The Gumbel distribution is also known as the Smallest C. Probability distributions Extreme Value (SEV) distribution (Type I) and has a positive Triangular distribution location parameter µ and a positive scale parameter β. A The triangular distribution is bounded on the left and right random variate, T, for the Gumbel distribution was calculated and can be symmetric or skew depending on the values of the using (4) and the RandUniform(0,1) function to generate a three parameters that determine the shape of the distribution. random number. This distribution is mainly used in the The three parameters of the triangular distribution are analysis of extreme values and in survival analysis. typically: , , ∙ (4) a = lower bound (minimum) m = most likely value (mode) where ρ is a random number. b = upper bound (maximum) Frechet distribution The density and cumulative distribution functions for the The Fréchet distribution, also known as the inverse triangular distribution are not provided by MS Excel but can Weibull distribution, belongs to the broader family of be calculated using the formulas from [3]. The random extreme value distributions and has a positive shape variates for the triangular distribution are also not provided in parameter
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